1. Field
The present invention relates to a system and method for harvesting energy from environmental vibrations. More particularly, embodiments of the present invention relate to a system and method for harvesting energy by applying mechanical strain to a specially-shaped piezoelectric element and harvesting the resulting output power using a combination of a voltage inversion circuit, a voltage compensating circuit, and the specially-shape piezoelectric element.
2. Description of the Related Art
Wireless sensors and other portable electronic devices are becoming ubiquitous. Batteries are commonly used to power these devices, but due to the batteries' low durability, they must be replaced periodically. This procedure is costly, especially for devices that are associated with remote structures. Examples of such devices include health monitoring of earthmoving equipment and bridges where continuous data gathering is essential. Accordingly, there is a need for a more durable source of energy beyond batteries.
A source of energy previously studied and applied is energy harvesting from environmental or ambient sources. The conversion of ambient energy into usable electrical form is often referred to as energy harvesting or scavenging. Scavenged energy can be obtained from, for example, solar, thermal, pyroelectric, and piezoelectric sources. Depending on the type of source of ambient energy, the scavenged energy might be quite large. However, in the case of piezoelectric sources, the scavenged energy is often quite small, and thus, the harvested or scavenged energy is only useful either as a supplement to an existing energy source, such as a battery, or to power small electronic devices. Harvested energy from piezoelectric sources can also be used to extend the lifetime of installed batteries and/or reduce the needed size of batteries, which is an important consideration for mobile devices.
Prior art techniques of harvesting energy from piezoelectric sources have relied on small, cantilever beams using a piezoelectric element that can scavenge power from low-level ambient vibration sources. The cantilever beams are designed to resonate as close as possible to the frequency of the driving surface on which they are mounted. An average power consumption of 120 microwatts is common. Prior art scavenging techniques have mainly focused on finding an efficient mechanical structure to transfer maximum possible strain to the piezoelectric element.
One possible design for transferring mechanical strain to a piezoelectric element is based on coupling the piezoelectric element to a two-layer bender (or bimorph) mounted as a cantilever beam. The bender's top and bottom layers are directly coupled with or comprised of the piezoelectric material. By choosing an appropriate thickness of the piezoelectric material, an inner layer of the beam improves overall electromechanical coupling. Additionally, the shape of the beam itself, and consequently, the shape of the piezoelectric material coupled to the beam, affects the possible power output. This is because some shapes are more efficient at exploiting the mechanical strain on the beam than other shapes.
Accordingly, there is a need for a cantilever beam design that (a) maximizes the piezoelectric response for a given input; (b) improves scavenger robustness by reducing stress concentration on the beam; and (c) minimizes power loss (damping) associated with the beam's shape or design.
Prior art systems and methods for increasing the energy harvested also use circuitry to boost the power. One common prior art circuit for boosting power from a piezoelectric element is a voltage inversion circuit, commonly referred to as synchronized switch harvesting on inductor (“SSHI”). The voltage inversion circuit injects current into a clamping capacitor of the piezoelectric element. Different voltage inversion techniques have been proposed to shape the voltage waveform in order to increase the voltage and power factor from the piezoelectric element, which consequently increases the harvested power. The shaped voltage waveform in prior art voltage inversion techniques increases the harvested power noticeably; however, the optimal voltage waveform is usually not obtained. Accordingly, a system and method for shaping and achieving the optimal voltage waveform in piezoelectric power harvesting is needed.